Modal Block: add an option to display the Modal Close Button at the bottom of the modal

The current Modal Close Button is at the bop of the modal block, which is contrary to users' habits. Normally, when we click open the popup, we will scroll to browser the content till the bottom, then when we want to close the popup, we will need to scroll back to the top again. (I know that modal block can be closed by clicking its outside area, but some people may not know this, they will scroll back to the top again, which is not very user-friendly). A lot of sites are putting the Close Button at the bottom of the modal, which is close to people's reading habits to popus.